In general, the present invention relates to aerial tramways, and more particularly, relates to detachable and permanent grip assemblies used to secure passenger or cargo carrier units of an aerial tramway to the haul rope.
As used herein, the expression "aerial tramway" shall be understood to include any haul rope based conveying system of the type having a plurality of passenger or cargo carrier units (chairs, gondolas, cabins, platforms, etc.) secured to a haul rope to enable those units to be conveyed along a path. Aerial tramway haul rope grip assemblies generally can be categorized into two groups, namely, detachable grip assemblies and permanently-affixed grip assemblies. "Detachable" grip assemblies include a grip actuating mechanism which can be operated at a tramway station to overcome gripping forces generated in the gripand release the grip for detachment from the haul rope. Detachable grips are removed regularly from the tramway haul rope to permit detachment of the carrier unit from the haul rope for loading and unloading of the passenger or cargo carrier units are tramway terminals or stations.
Permanently-affixed grip assemblies typically are used in aerial tramways, such as chair lifts and ski lifts. Although the permanently-affixed grips can be removed from the haul rope, they seldom are removed unless the grips are undergoing maintenance or repair. "Permanently-affixed" grip assemblies lack a machanism for release of the grip as the haul rope and passenger carrier units are advanced through the tramway loading and unloading stations.
The performance criteria for aerial tramway grip assemblies typically have been established by industry regulating bodies or the laws of various countries. In the United States, for example, the gripping force generated by a grip assembly must be at least three times that required to prevent slippage of the grip along the rope, and the grip must produce a 3% reduction in the rope area at the grip. Since rope manufacturers typically manufacture haul ropes to a nominal diameter plus 6% minus 3% along the rope length, detachable grip assemblies must be capable of meeting these performance criteria notwithstanding variation of the rope diameter along the length of the rope.
In recent years there also has been a trend in the aerial tramway industry to increase the uphill carrying capacity of tramways. This, in turn, has resulted in an increase in prime mover horsepower and a corresponding increase in haul rope diameter. Carrier units also have increased in capacity and, accordingly, weight. Thus, the rope gripping forces are now quite substantial and the problems attendant to detaching and attaching the carrier units from the rope are substantial. A gondola system having a spring-biased movable jaw may require, for example, 5000-6000 pounds of force to open the grip.
Accordingly, both permanently-affixed and detachable tramway grip assemblies must be made as fail-safe as possible, but the problems associated with making detachable grip assemblies fail-safe generally are greater than for permanently-affixed grip assemblies. The adverse environmental conditions in which tramways typically operate, plus the repeated need for opening and closing of the grips while heavily loaded, can result in fatiguing and failure of the detachable grip components, such as jaws, biasing springs and/or lever arms used to apply the gripping force to the jaws. Moreover, the increased loading of tramway systems makes grip failures potentially more catastrophic.
U.S. Pat. No. 3,037,464 to Penney, et al. is typical of a "permanently-affixed" grip assembly. The Penney, et al. grip includes a movable jaw which is biased toward a fixed jaw by two compression springs. The use of two independent springs to bias the same gripping jaw is broadly pertinent to the gripping assembly of the present invention in that such redundancy provides one form of fail-safe protection against grip release. If, for example, one of the two springs should fail, the remaining spring will still apply a gripping force to the haul rope. If either gripping jaw fails, however, the grip will release the haul rope.
In general, detachable tramway grip assemblies with their grip release mechanisms tend to be relatively complex. Detachable grips may have over 100 parts and require involved rail support structures at the tramway stations to support the grips as they are operated while moving with the haul rope. A typical detachable aerial tramway grip assembly will be sold at a price of $1500 to $1700 as a result of this complex structure. Since the number of grip assemblies in a system is large, the total cost of a tramway system is significantly increased by the grip assembly costs. Complex grip opening and closing grip support structures further increase the tramway system cost, and grip assembly complexity inherently increases repair and maintenance costs.
Typical of prior art detachable tramway grip assemblies currently in use are the grips shown in French Patent No. 1,453,517to Pomagalski and in U.S. Pat. No. 3,685,457 to Wallmannsberger. The Pomagalski grip assembly includes a fixed jaw and a movable jaw which is spring-biased toward the fixed jaw. An actuating lever is provided to overcome the spring force and permit opening of the grip assembly at the tramway terminals. The Pomagalski grip, however, requires a complex set of support rails to effect opening of the grip on the move, and if either gripping jaw should fail, the grip will release the haul rope. Since there are two biasing springs, this grip assembly does provide redundancy against the failure of one of the springs, but it is not fail-safe if either of the haul rope gripping jaws fail. Moreover, the entire gripping force is generated by the biasing springs.
In U.S. Pat. No. 3,685,457, to Wallmannsberger a grip assembly is disclosed in which two side-by-side movable jaws are spring biased toward a stationary jaw or surface to grip the haul rope therebetween. This detachable grip provides redundancy in that the two springs are independent of each other so that failure of one spring does not cause failure of the entire grip. As was the case in the Pomagalski grip, the gripping force in the Wallmannsberger grip is generated solely by the biasing springs. Thus, the stationary jaw or surface merely reacts to the gripping force generated by the movable jaw; the stationary jaw does not add to the gripping force on the haul rope. Moreover, failure of either jaw will release the gripping force on the haul rope.
German Patent No. 1,131,718 to Pohlig discloses a detachable tramway grip assembly which employs two opposed, movable, spring-biased gripping jaws. The gripping jaws are at one end of pivotally mounted levers while the biasing spring is proximate an opposite end of the levers. In one form of the Pohlig '718 grip, the two movable jaws cooperate with a stationary support surface to grip the haul rope. The Pohlig '718 grip assembly, however, is no failsafe. If either jaw, eitherlever or the spring should fatigue and fail, the grip will release the haul rope. As is true of the Pomagalski grip, the entire gripping force in the Pohlig '718 grip assembly is generated by biasing spring.
U.S. Pat. No. 3,827,372 to Laurent is similar in structure to Pohlig '718 except that the two movable jaws are biased by torsion bars instead of a compression spring. The entire gripping force is generated by the torsion bars, and failure of a jaw or either torsion bar will cause failure of the grip.
German Patent No. 308,544 to Blerchert discloses several forms of detachable tramway grip assemblies. In each case, however, failure of the biasing spring, failure of either jaw, or failure of either lever on which the haul rope gripping jaws are carried, results in release of the gripping force and failure of the overall assembly.
Detachable aerial tramway grip assemblies also have been devised which employ opposed converging surfaces which are gravity biased to wedge against the haul rope and thereby effect gripping. One such prior art detachable grip assembly is shown in U.S. Pat. No. 4,364,314 to Gaudet. The Gaudet grip assembly does not employ movable spring biased jaws. Thus, all the gripping force is achieved through a wedging action. This approach poses problems as the carrier unit weight increases. Thus, unwedging of the grip from the haul rope can become difficult. Additionally, should either opposed surface of the haul rope gripping clips fatigue and fail, that clip will fail. Since there are two clips, the grip assembly does contain a degree of redundancy.
Finally, in German Patent No. 542,702, also to Pohlig, a grip assembly is disclosed in which the haul rope is engaged by two shoes which essentially ride the top of the haul rope. The shoes include two opposed cylinders with grooves that are skewed to match the lay of the haul rope strands. The Pohlig '702 grip is designed to permit free rotation of the haul rope about its longitudinal axis while gripped by the rotatable grooved cylinders. This grip assembly makes no use of a resiliently biased movable jaw as a part of the gripping mechanism, and any wedging or binding between grooved cylinders is minimal since the cable can rotate. Additionally, detachment of the grip assembly "can be accomplished smoothly and free of shocks," which suggests little or no wedging action.